1. Field of the Invention
The invention relates generally to methods for providing improved drill bits. In particular, the present invention relates to methods for generating localized and/or asymmetrically graded compositions in cutting elements.
2. Background Art
Roller cone rock bits and fixed cutter bits are commonly used in the oil and gas industry for drilling wells. FIG. 1 shows one example of a conventional drilling system drilling an earth formation. The drilling system includes a drilling rig 10 used to turn a drill string 12, which extends downward into a well bore 14. Connected to the end of the drill string 12 is roller cone-type drill bit 20, shown in further detail in FIG. 2.
As shown in FIG. 2, a roller cone bit 20 typically comprises a bit body 22 having an externally threaded connection at one end 24, and a plurality of roller cones 26 (usually three as shown) attached to the other end of the bit body 22 and able to rotate with respect to the bit body 22. Attached to the roller cones 26 of the bit 20 are a plurality of cutting elements 28, typically arranged in rows about the surface of the roller cones 26. The cutting elements 28 can be inserts, polycrystalline diamond compacts, or milled steel teeth. If the cutting elements 28 are milled steel teeth, they may be coated with a hardfacing material. One particular type of insert uses tungsten carbide and thus are known as TCI.
Many factors affect the durability of a TCI bit in a particular application. These factors include the chemical composition and physical structure (size and shape) of the carbides, the chemical composition and microstructure of the matrix metal or alloy, and the relative proportions of the carbide materials to one another and to the matrix metal or alloy.
Many different types of tungsten carbides are known based on their different chemical compositions and physical structure. Three types of tungsten carbide commonly used in manufacturing drill bits are cast tungsten carbide, macro-crystalline tungsten carbide, and cemented tungsten carbide (also known as sintered tungsten carbide).
Cemented carbides, as exemplified by WC—Co, have a unique combination of high elastic modulus, high hardness, high compressive strength, and high wear and abrasion resistance with reasonable levels of fracture toughness. See Brookes, Kenneth J. A., “World Directory and Handbook of Hardmetals and Hard Materials,” International Carbide Data, 1997. This unique combination of properties makes them ideally suited for a variety of industrial applications, such as drill bits. See “Powder Metal Technologies and Applications, Powder Metallurgy Cermets and Cemented Carbides, section on Cemented Carbides,” Metals Handbook, Vol. 7, ASM International, Metals Park, Ohio, 1998, pp. 933-937. The very high modulus of WC, its ability to plastically deform at room temperature, excellent wetting of WC by cobalt, good solubility and reasonable diffusivity of W and C in cobalt, retention of the face centered cubic form of cobalt in the as sintered condition all contribute to this versatility.
Attempts to develop alternate cemented carbide systems that can provide higher levels of fracture toughness for a given hardness (resistance to wear) have only resulted in limited success. These alternate materials often find niche applications but lack the versatility of WC—Co. See Viswanadham et al., “Transformation Toughening in Cemented Carbides, I. Binder Composition Control”, Met. Trans. A. Vol. 18A, 1987, p. 2163; and “Transformation Toughening in Cemented Carbides, II. Themomechanical Treatments”, Met. Trans. A., Vol. 18A, 1987, p. 2175.
Property changes in WC—Co and other similar systems are often accomplished by variations in binder contents and/or grain sizes. Higher binder contents and larger grain sizes lead to increased fracture toughness at the expense of wear resistance (hardness), and vice versa. This inverse relationship between the wear resistance and fracture toughness of these materials makes the selection of a particular cemented carbide grade for a given application an exercise in compromise between resistance to wear and resistance to catastrophic crack growth.
Over the years, many attempts have been made to increase the fracture resistance of WC—Co without sacrificing wear resistance. Two approaches have produced successful results: (1) producing surface compressive stresses through mechanical means; and (2) producing dual-property cemented carbides by carburizing carbon-deficient cemented carbides (WC—Co) having uniformly distributed eta carbide. The mechanically imposed compressive stresses increase the apparent fracture toughness with essentially no change in wear resistance. Dual-property carbides, such as the DP™ carbides from Sandvik AB Corporation (Sandviken, Sweden), have carbon gradients near the surface during processing, which result in binder (Co) depletion near the surface that results in significant residual surface compressive stress. The high level of compressive stress results in an increase in the apparent fracture toughness of the material, while the wear resistance also increases due to lower binder contents near the surface.
While these prior art treatments are capable of producing improved inserts, they are applied to the entire insert and are not suitable for localized variations in material properties of an insert (cutting element). Therefore, there still exists a need for methods that can provide localized variations in material properties in an insert.